Reactive gases and gas mixtures are used in many industrial operations, especially the processing of semiconductor wafers in the fabrication of integrated circuits. The gases may be used, for example, in etching the wafers and processing the photoresist. Because of the extremely fine tolerances necessary, the impinging gas stream must be as uniform as possible to uniformly process the wafer. In addition, the process should take place in the shortest possible time to maximize production rates.
There are at present two basic methods of producing the reactive gases. Both methods employ an RF field, preferably in the microwave range, to couple energy to a raw gas to produce the excited gas primarily in the form of ions and free radicals. One method generates the excited gas by passing the gas through a dielectric tube extending transversely through a microwave guide. The excited gas flows out of the waveguide to a reaction chamber in which the wafer is placed. An example of this method is disclosed in U.S. Pat. No. 4,207,452.
The other method employs a flat cylindrical plasma-production chamber mounted in a waveguide. The raw gas is introduced into the chamber to form a thin disc of the excited gas. The wafer to be processed may then be mounted in the chamber, or the excited gas may be drawn from the chamber into a downstream reaction chamber. An example of this method is disclosed in U.S. Pat. No. 4,507,588.
Both of the methods have drawbacks which limit their utility for wafer processing. The method of U.S. Pat. No. 4,207,452 is a good source of radicals but a poor source of ions due to the recombination of ions on the inside surface of the dielectric tube. The low gas volume to tube surface area ratio coupled with the tube length creates a high probability that the ions produced in the waveguide will contact a wall before they impinge on the wafer being processed. As the ions collect on the walls, they tend to recombine into molecules, destroying their utility as excited gas species. Thus, the patented method is only practically useful as a low volume, small area radical source.
The method of U.S. Pat. No. 4,507,588 also exhibits a wall recombination problem due to the low gas volume to surface area ratio. Primarily, however, the method is inefficient; a large amount of raw gas and microwave power is needed to create a sufficient volume of excited gas. This inefficiency is primarily due to the size and shape of the plasma; the disc provides a very short gas residence time in which most of the gas flows out of the disc before it becomes excited.
The volume of the plasma should ideally be as large as practicable to efficiently create the reactive gas, which may include atoms, free radicals, excited species, ions and/or electrons. The reactive gas may also be used as a light source. The large volume is needed because the gas molecules must, on average, move within the plasma production area a distance known as the mean free path before the gas molecules become sufficiently excited by the electron cloud. If the size and shape of the plasma does not provide a sufficient free path distance, a low number of molecules will achieve the desired excitation state, and the resultant reactive gas will be relatively weak, causing the wafer processing to take place very slowly. In addition, the microwave power and raw gas are inefficiently used; the plasma production volume per unit of gas and power is low.
Plasmas having a large surface area relative to their volume and/or available free path distance are also relatively inefficient due to wall effects. Ions and free radicals tend to adhere to container walls, increasing their residence time and so the likelihood of their recombining, which destroys the plasma. It has been estimated that there is a seventy-five (75) to one hundred (100) percent chance of recombination when the plasma ions encounter a surface. Thus, to maximize reactive gas creation efficiency, it is desirable to create a large volume of plasma with a relatively small surface area.